Feedback usually generates a loud howling sound in the output transducer. Detectors which may detect the presence of feed back are used on order to control the development of feed back in feedback cancellation systems where the detection may be used to control the adaptation, e.g. increase adaptation speed at the presence of howl. It is also used in hearing aids without feedback cancellation system as an “online feedback manager”. The gain is then reduced at the frequency where howl is detected. The feedback is usually a rather narrow-band and loud signal, and it is not difficult to arrange a detector for such a signal in the signal path, however should a similar signal occur in the environment the detector will react to such a signal as well as to the real feedback signal. The problem to be solved by the invention is to distinguish tones caused by feedback from environmental sounds like tones, e.g. flute or other sounds with a high content of pure tones.
Patent application PCT/2004/000701 describes a method to detect howl that comprises analysis of the phase of the signal in the frequency domain. Successive windows of the input signal are then converted into frequency bands in the frequency domain. The change of phase of successive windows is then compared and if it is constant it is assumed to be caused by howl. The present invention differs from the application PCT/2004/000701 as the present invention modifies the phase of the forward path of the hearing aid and looks for changes in the frequency of the howl that can be expected if the howl is caused by feedback.
Patent application PCT/2004/000702 describes a method to suppress feedback that involves modification of the phase of the forward path. The phase is then modified in a random pattern, in opposite to the present invention that uses a predefined change of the phase. Further, the present invention is a method to detect howl and not to suppress feedback as application PCT/2004/000702.